1. Field of the Invention
The present invention relates to a data communication system having a channel switching means, for example, a radio data communication system provided with channel switching equipment between a main channel and a protection channel.
2. Description of the Related Art
Usually, a channel switching means which is employed in a data communication system uses both a main channel and protection channel. That is, when the transmission quality of the main channel deteriorates, and accordingly a data error rate increases, a transmitting terminal station automatically switches the transmission line from the main channel, which is transmitting data to a receiving terminal station, to the protection channel, which is transmitting auxiliary data thereto. To cope with this, the receiving terminal station selects data received via the protection channel instead of the main channel, and outputs the same as main channel data.
The above-mentioned channel switching can be performed without even an instantaneous interruption in the flow of data if the above-mentioned transmission quality deteriorates slowly. However, if the transmission quality deteriorates quickly, it is very liable to result in an instantaneous interruption in the flow of data. Thus, in the field of data communication systems, it has been hoped to prevent such an instantaneous interruption in data flow caused by channel switching when rapid deterioration of the transmission quality occurs due to, for example, sudden fading.
In the prior art, as will be exemplified in detail hereinafter, there is an inevitable loss of data during the short transition period when switching from the main channel to the protection channel. Thus, the prior art channel switching means used in data communication systems produces a problem in that there is a high probability of a data error occurring when there is a rapid deterioration in transmission quality.